With the proliferation of wireless communications applications, the wireless industry is focused on more advanced techniques for higher data throughput to more users in wireless systems. One method to increase spatial re-use of wireless spectrum is Multiple-Input and Multiple-Output (MIMO) antenna systems, especially Multi-User MIMO (MU-MIMO). In a wireless communication system, a wireless node with multiple antennas, be it a Base Station (BS) or a User Equipment (UE), can use beamforming in downlink (DL) or uplink (UL) to increase the Signal-to-Noise Ratio (SNR) or Signal-to-Interference-plus-Noise Ratio (SINR), hence the data rate, of the links with other wireless nodes. MU-MIMO can beamform to multiple UEs simultaneously in a frequency and time block, e.g., a Resource Block (RB), i.e., using spatial multiplexing to provide capacity growth without the need of increasing the bandwidth. A BS may be equipped with a large number of antennas, e.g., many tens to hundreds, to serve many users, referred as massive MIMO systems [1].
A relay that instantly amplifies and forwards an in-band signal (hereafter referred to simply as relay) is an efficient component to increase the SNR of UEs with bad channel condition, and improve channel estimation accuracy. Although a MIMO BS with a large number of antennas can extend its DL coverage range through beamforming, the SINR of UEs can decay quickly as the distance to a UE increases or due to shadowing. Hence, the UEs far away from the BS have significantly lower SINRs than those close to the BS, due to large scale fading, shadowing, among other factors. In addition, the UL range, and hence the UL channel estimation accuracy, is limited by the transmitting power of UEs. With relays, the BS is able to receive the signals with high SNR from the UEs even close to the cell edge.
The channel estimation is a challenging and important issue in multi-user MIMO systems. In order for the BS to beamform to multiple UEs using the plural of antennas, the BS needs to know sufficiently and accurately the DL channels to the UEs, e.g., the DL Channel State Information (CSI) of each UE. However, it is not efficient to obtain the DL CSI directly by sending reference pilots in the DL because of two reasons: (1). The large number of antennas on the BS would cause large system overhead for reference signals in the downlink; (2). Dozens of bits are needed to quantize the CSI accurately, which causes overload of the feedback channel in the UL. Fortunately, the reciprocal property of an over the air wireless channel, such as in a Time-Division Duplexing (TDD) system or in an Frequency-Division Duplexing (FDD) system using switching to create channel reciprocity as described in our PCT application PCT/US14/71752 filed on Dec. 20, 2014, claiming the benefit of provisional patent application 61/919,032 filed on Dec. 20, 2013, can be employed to reduce the channel estimation overhead. In such a system, a UE sends a pilot signal, e.g., Sounding Reference Signal (SRS), that is received by all the antennas on the BS in the UL. The BS estimates the UL CSI through the received pilot signal and uses it to estimate the DL CSI based on channel reciprocity.
One embodiment of this invention is a symmetric relay that will allow efficient channel estimation using wireless channel reciprocity. With relays in the systems, the Total Channels (TCs) between BS and UEs comprise of channels through the relays and the channels without the relays. To efficiently estimate the channel through the UL CSI based on channel reciprocity, the channels through the relays also need to be reciprocal. Only with symmetric design of relays, the TCs in the UL and DL will be the same, and then the UL CSI can be used to estimate the DL TCs. The BS can use the channel estimates of the TCs to compute precoding matrices or detection matrices; and use the precoding or detection matrices to perform beamforming to transmit data to or receive data from multiple UEs using the same frequency resources, whereas MU-BF can use any known or future MU-BF method, e.g., Zero-Forcing (ZF), Regularized ZF (RZF), Conjugate Beamforming (CB), Minimum Mean Square Error (MMSE), Dirty Paper Coding (DPC), etc.
There have been some prior works considering relays in wireless systems. In [2], Boris Rankov and Armin Wittneben described relay-assisted wireless MIMO channels for single-user MIMO (SU-MIMO) where the destination antennas are equally spaced in a linear array, and the relays are limited to single antenna nodes. The relays use time division duplex (TDD), i.e., receive a data packet in one time slot and transmit it in another time slot. This reduces the spectral efficiency and requires synchronization of the relays. In [3] Chae, Chan-Byoung, et al. descried MIMO relaying with linear processing for multiuser transmission in fixed relay networks that is also TDD two-hop communication, same as in [2]. In [4] Wei Xu and Xiaodai Dong described a limited feedback design for MIMO-relay assisted cellular networks with beamforming, in which each UE is required to feed back its quantized CSI to the relay, and the relay sends the quantized beamforming vectors to the BS. In addition, it is limited to the oversimplified case of (Number of antennas of the BS)=(Number of antennas of the relays)=(Number of antennas of the UEs). In [5] Mats Andersson and Bo Goransson proposed a MIMO system with repeaters for beam steering not beamforming, that is, it is limited to steering antenna lobes of repeaters to aim towards a UE, instead of real beamforming at the BS.
None of the prior art contained symmetric relays. This invention described novel symmetric relays designs, which realize the channel reciprocity in UL/DL channels, and consequently enable the efficient channel estimation at the BS using UL pilots for DL beamforming to multi-users.
Another important parameter of a relay is high isolation between transmitting antennas and receiving antennas. Active cancellation may be required in some applications. There was no prior art for efficient amplify and forward relays using active cancellation.
Note that only key components are shown in the drawings, other components such as filters, terminators are neglected, but a person skilled in the understands the use of such components.